id Software releases Quake 1 source code under the GPLDec 22, 1999, 06:00 (7 Talkback[s])
[ Thanks to Ravi Swamy for this link. ]
From the q1source.txt file on the id site:
"This is the complete source code for winquake, glquake, quakeworld, and glquakeworld."
"The projects have been tested with visual C++ 6.0, but masm is also required to build the assembly language files. It is possible to change a #define and build with only C code, but the software rendering versions lose almost half its speed. The OpenGL versions will not be effected very much. The gas2masm tool was created to allow us to use the same source for the dos, linux, and windows versions, but I don't really recommend anyone mess with the asm code."
"The original dos version of Quake should also be buildable from these sources, but we didn't bother trying."
"The code is all licensed under the terms of the GPL (gnu public license). You should read the entire license, but the gist of it is that you can do anything you want with the code, including sell your new version. The catch is that if you distribute new binary versions, you are required to make the entire source code available for free to everyone."
The full text of the file may be found here.
The source code (in .zip format) may be found here.